For example a user has a non-custodial wallet at Trust wallet app
I've seen far too many topics about frozen funds in that wallet to believe they're non-custodial.
If you want an SVP wallet, use Electrum.
However those " light " wallet providers still uses dedicated or self hosted nodes, which means that they can still track the users IP and also when trying to update the balance of a specific seed/wallet, the amount held under an adress?
If you don't want anyone to know your IP address, use a VPN or Tor. If you don't want anyone to know which addresses belong to the same wallet, don't use an SVP wallet.
Also how privacy focused are those wallets then, if a surveillence firm can track a broadcasted transaction back to those wallet providers, because they use their own nodes?
If you're concerned about privacy, it's better to use Bitcoin Core (through Tor).
Also can the app or the Wallet provider also link different generated seeds on the same device back to the user or are they seperated from each other even if the same app on the same device was used for those different seed phrases?
If you use those wallets from the same IP address, it's not that hard to link them together. But it's very easy to avoid all this: don't do it.
What is the better solution for an mobile application to use?
Mobile wallets are hot wallets, you shouldn't trust them with any amounts larger than what you can afford to lose in the first place. I'd go for Mycelium or BlueWallet on mobile, but much of this depends on personal preference. Phones are designed to spy on you, so if you want privacy, it's better to get a(n old) laptop (with Linux).